Three-dimensional multi-gate devices, e.g., FinFETs or Tri-gate devices, are main-stream devices in current sub-20 nm technology. Such devices can effectively improve gate control capability and meanwhile suppress current leakage and Short Channel Effect (SCE).
For example, compared with a conventional single-gate bulk Si or SOI MOSFET, a dual-gate SOI MOSFET can suppress the SCE and Drain Induced Barrier Lowering (DIBL) effect more effectively. It also has a lower junction capacitance and is suitable for light channel doping. The dual-gate SOI MOSFET may have a threshold voltage adjustable by setting a work function of its metal gate, thereby obtaining a drive current of about two times larger than that of the conventional single-gate bulk Si or SOI MOSFET. This allows relaxation of a requirement on Effective Oxide Thickness (EOT). The tri-gate device has a gate covering a top surface and two side surfaces of a channel region of the device, which enables stronger gate-control capability than that of the dual-gate device. Gate-all-around nanowire multi-gate devices may have more advantages.
The gate-all-around nanowire device may have better gate-control capability and may suppress the SCE more effectively, thus exhibiting more advantages in size-reduction of sub-14 nm technology. However, it cannot provide greater drive current in an equivalent silicon plane area.
For example, for a device having an equivalent line-width of 1 μm, the gate-all-around nanowire device should have dimensions satisfying d*n+(n−1)*s=1 μm and π*d*n>1 μm, wherein d represents a diameter of a single nanowire, n represents a number of the nanowires, and s represents a pitch, i.e., distance, between two adjacent nanowires. For the diameter d being 3 nm, 5 nm, 7 nm, and 10 nm, s should be smaller than 6.4 nm, 10.6 nm, 15 nm, and 21.4 nm, respectively. That is, the lateral arrangement of the nanowire device should be very compact in order to obtain a gate width equivalent to a gate width of 1 μm of a bulk silicon device. Existing FinFET exposure and etching technology enables a pitch of about 60 nm between two adjacent fins, which is difficult to implement a three-dimensional arrangement of nanowires with a very small pitch.
An effective way to improve the drive current of a transistor is to implement a stack of gate-all-around nanowire structures in a perpendicular direction. However, the process or manufacturing method for such a structure is very difficult. Great challenge exists in compatibility with conventional processes and also in cost reduction. For example, an existing method comprises Si/SiGe multilayer hetero-epitaxy and selective etching. That is, a plurality of Si/SiGe stacks are hetero-epitaxially grown alternatively on a buried oxide (BOX) layer. Then SiGe layers are removed selectively by, e.g., wet etching, to obtain a stack of Si nanowires. This method is limited to quality of epitaxial layers and is high in cost. On the other hand, in a unit footprint area, a nanowire stack of a conventional structure, in which each nanowire is surrounded by a HK/MG gate stack, has a small effective overall current. In the same projection area, a fin of non-stacked nanowires has a larger conductive effective cross-section, which is perpendicular to an extension direction of the fin or nanowires, i.e., to a channel direction.
Therefore, it is desired to find a device structure that can increase an effective width of a conductive channel and the drive current as well as a method for manufacturing the same.